Changes Affecting the Equilibrium Constant | CIE AS Chemistry Revision Notes 2025

Changes that Affect the Equilibrium Constant

If all other conditions stay the same, the equilibrium constant K_c is not affected by any changes in concentration of the reactants or products
For example, the decomposition of hydrogen iodide:

2HI ⇌ H₂ + I₂

K_c = $\frac{[H_{2}] [I_{2}]}{{[HI]}^{2}}$ = 6.25 x 10^–3

A change in pressure only changes the position of the equilibrium (see Le Chatelier’s principle)
If all other conditions stay the same, the equilibrium constant K_c is not affected by any changes in the pressure of the reactants and products

2HI (g) $⇌$ H₂ (g) + I₂ (g) K_c = $\frac{[H_{2}] [I_{2}]}{{[HI]}^{2}}$

With an increase in temperature:
- [H₂] and [I₂] increases
- [HI] decreases
- Because [H₂] and [I₂] increase and [HI] decreases, the equilibrium constant K_c increases

2SO₂ (g) + O₂ (g) $⇌$ 2SO₃ (g) K_c = $\frac{{[{SO}_{3}]}^{3}}{{[{SO}_{2}]}^{2} [O_{2}]}$

With an increase in temperature:
- [SO₃] decreases
- [SO₂] and [O₂] increases
- Because [SO₃] decreases and [SO₂] and [O₂] increase, the equilibrium constant K_c decreases

If all other conditions stay the same, the equilibrium constant K_c is not affected by the presence of a catalyst
A catalyst speeds up both the forward and reverse reactions at the same rate so the ratio of [ products ] to [ reactants ] remains unchanged

Factors affecting K_c

An equilibrium is established in the following reaction:

AB (aq) + CD (aq) $⇌$ AC (aq) + Bd (aq) ΔH = +180 kJ mol^-1

Which factors would affect the value of K_c in this equilibrium?

Answer

Only a change in temperature will affect the value of K_c
Any other changes in conditions would result in the position of the equilibrium moving to oppose this change
Adding a catalyst increases the rate of reaction meaning the state of equilibrium will be reached faster but has no effect on the position of the equilibrium and, therefore, K_c is unchanged

Factors which increase K_p value

What will increase the value of K_p for the following equilibrium?

2A (g) + B (g) $⇌$ 2C (g) ΔH = +6.5 kJ mol^-1

Answer

Only temperature changes permanently affect the value of K_p
An increase in temperature shifts the reaction in favour of the products
The [ products ] increases and [ reactants ] decreases, therefore, the K_p value increases